Re: [Paddlewise] Breaking Waves Issue?

The best reference for waves is "Waves and Beaches" by Willard Bascom. 
In this book he talks about deep-water and shallow-water waves.

Bascom says: "A shallow-water wave is one that is traveling in water 
whose depth is less than half the wave length; that is, if the depth of 
water is small compared to the wave length, the effect of the bottom is 
sufficient to alter substantially the character of the waves."

What this says to your question is that you should first look at the 
wave length, not the height, as it is the wave length that determines 
if the proximity of the bottom will change the character of the wave 
from non-breaking to breaking. For example if you have a wave that is 1 
foot tall in the middle of the Pacific, but has a wave length of a 
hundred miles, you should head for the hills, you have a tsunami on the 
way.

But for our purposes, we don't really care what the wave height is out 
in the middle of the ocean. We are typically interested in waves which 
are already shallow water. In shallow-water the deep-water wave slows 
down. As a consequence the wavelength reduces (the faster deep-water 
wave catches up with the slower shallow-water wave as it approaches 
shore). As the wavelength decreases, the steepness of the wave 
increases. As the wave crest enters water that is about twice the wave 
height, the crest starts rising higher, further increasing the wave 
steepness. According to Bascom, "Finally, at a depth of water roughly 
equal to 1.3 times the wave height, the wave becomes unstable." Then it 
breaks. I read this as the wave height just prior to breaking will be 
the determining factor, not the original deep-water wave height. I 
don't think you can predict the breaking depth of a deep-water swell 
based on it's height alone. A 1' high wave with a wave length of 7 feet 
will break in about 18" of water, but a 1' high wave with a wave length 
of 300 feet will break in much deeper water.

But if you see a 10' wave coming at you and you are in 13' of water, 
you might want to move.
Nick


On Mar 9, 2004, at 10:42 PM, Strosaker wrote:

> Paddlewisers,
>
> I recently purchased and have been enjoying reading Sea Kayaker 
> Magazine's
> Handbook of Safety and Rescue by Doug Alderson and Michael Pardy. 
> However,
> something seemed unusual on page 48 as I read, "The crest will rise and
> break when the depth of the water is approximately three-quarters of 
> the
> swell height. A 4-foot swell will break in water 3 feet deep. The 
> height of
> the wave at the time it breaks will be approximately 1.5 times its 
> original
> deep-water height. The 4-foot swell will rise to a 6-foot crest before
> breaking in 3 feet of water."
>
> I then checked a couple of other books I've read, and the information
> conflicted. Derek Hutchinson in The Complete Book of Sea Kayaking 
> wrote,
> "When the depth is approximately one-and-a-half times the wave height, 
> the
> wave begins to break." Nigel Foster in Nigel Foster's Surf Kayaking 
> wrote,
> "The wave will steepen until the water is 1.3 times the height of the 
> wave
> at that time, when it will break."
>
> Foster and Hutchinson's 1.3 to 1.5 times the depth of the water is not 
> a big
> difference, but Alderson and Pardy's .75 times is a big difference. 
> Maybe
> the real difference here is that Alderson and Pardy are referring to 
> swells,
> while Foster and Hutchinson are referring to waves. If that is the 
> case,
> Alderson and Pardy's number is .5 times when referring to waves and is 
> even
> a bigger difference.
>
> Is there something else going on here that I am missing?
>
Nick Schade

Guillemot Kayaks
824 Thompson St
Glastonbury, CT 06033
USA
Ph/Fx: (860) 659-8847
http://www.guillemot-kayaks.com/
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